Fig. 4. Control treatments do not influence the central zone. (A-H) Laser ablations (yellow arrowheads) as in Fig. 2C, but at the periphery instead of the centre. (A) Laser ablation at the periphery approximately at the site of incipient leaf formation. (B,C) Consecutive video images of a single meristem with an ablation as in (A). Cut primordia are dark green, and the youngest primordia are highlighted in yellow for clarity. (B) Immediately after ablation (t₀); (C) 2 days after ablation. Leaf formation at the site of the ablation is suppressed (arrowheads), while two new primordia (I₁ and I₂) were induced at the next two expected positions. (D,E) Consecutive video images of a single meristem with an ablation as in (A). (D) t₀; (E) 2 days after ablation. I₁ is initiated closer to P₁ than normal, resulting in a divergence angle of approximately 90° instead of 137°. Arrowheads point to the lesion. (F) Ablation on P₁. 2 days after the ablation (arrowhead), the primordium has recovered and split in two halves. (G) In situ hybridisation with a LeWUS probe 6 hours after an ablation at the periphery. The LeWUS expression domain remained normal. (H) 2 days after ablation, the lesion was displaced (arrowhead), and LeWUS continued to be expressed in the normal area. (I-L) Effects 4 days after treatments of the CZ with stress metabolites and oxidants. (I) Control. (J) Treatment with 1 mM salicylic acid. (K) Treatment with 1 mM hydrogen peroxide. (L) Treatment with 0.1 mM Paraquat. Treatments did not affect the formation rate or the positioning of leaf primordia. Lanolin paste (red) remained in the centre, indicating that the growth centre persisted. P₅, P₄, P₃, P₂, and P₁ indicate the bases of pre-existing leaf primordia that were removed at the beginning of the experiment; I₁, I₂ and I₃ indicate primordia formed after the ablation. Scale bars: 100 µm.